Hamster Superior Colliculus Research Articles

Glutamate Receptors GluR1 and GluR4 in the Hamster Superior Colliculus: Distribution and Co-localization with Calcium-Binding Proteins and GABA

We investigated the distributions of AMPA glutamate receptor subtypes GluR1 and GluR4 in the hamster superior colliculus (SC) with antibody immunocytochemistry and the effect of enucleation on these distributions. We compared these labelings to those of GluR2/3 in our previous report (Park et al., 2004, Neurosci Res., 49:139–155) and calcium-binding proteins calbindin D28K, calretinin, parvalbumin, and GABA. Anti-GluR1-immunoreactive (IR) cells were scattered throughout the SC. By contrast, anti-GluR4-IR cells formed distinct clusters within the lower lateral stratum griseum intermediale (SGI) and lateral stratum album intermediale (SAI). The GluR1- and GluR4-IR neurons varied in size and morphology. The average diameter of the GluR1-IR cells was 13.00 µm, while the GluR4-IR cells was 20.00 µm. The large majority of IR neurons were round or oval cells, but they also included stellate, vertical fusiform and horizontal cells. Monocular enucleation appeared to have no effect on the GluR1 and GluR4 immunoreactivity. Some GluR1-IR cells expressed calbindin D28K (9.50%), calretinin (6.59%), parvalbumin (2.53%), and GABA (20.54%). By contrast, no GluR4-IR cells expressed calcium-binding proteins or GABA. Although the function of the AMPA receptor subunits in SC is not yet clear, the distinct segregation of the GluR subunits, its differential colocalization with calcium-binding proteins and GABA, and differential responses to enucleation suggest the functional diversity of the receptor subunits in visuo-motor integration in the SC.

Inhibitory Plasticity Facilitates Recovery of Stimulus Velocity Tuning in the Superior Colliculus after Chronic NMDA Receptor Blockade

The developing nervous system is shaped in important ways by spontaneous and stimulus-driven neural activity. Perturbation of normal activity patterns can profoundly affect the development of some neural response properties, whereas others are preserved through mechanisms that either compensate for or are unaffected by the perturbation. Most studies have examined the role of excitation in activity-dependent plasticity of response properties. Here, we examine the role of inhibition within the context of response selectivity for moving stimuli. The spatial extent of retinal input to the developing hamster superior colliculus (SC) can be experimentally increased by chronic NMDA receptor (NMDAR) blockade. Remarkably, stimulus velocity tuning is intact despite the increase in excitatory inputs. The goal of this study was to investigate whether plasticity in surround inhibition might provide the mechanism underlying this preservation of velocity tuning. Surround inhibition shapes velocity tuning in the majority of superficial layer SC neurons in normal hamsters. We show that despite the NMDAR blockade-induced increase in feedforward excitatory convergence from the retina, stimulus velocity tuning in the SC is maintained via compensatory plasticity in surround inhibition. The inhibitory surround increased in strength and spatial extent, and surround inhibition made a larger contribution to velocity tuning in the SC after chronic NMDAR blockade. These results show that inhibitory plasticity can preserve the balance between excitation and inhibition that is necessary to preserve response properties after developmental manipulations of neural activity. Understanding these compensatory mechanisms may permit their use to facilitate recovery from trauma or sensory deprivation.

햄스터 상구의 deeper layers에서 calretinin이 함유 신경세포

Calcium-binding protein calretinin is thought to play important roles in calcium buffering. Recently, we reported on the distribution, morphology of calretinin-immunoreactive (IR) neurons and the effects of eye enucleation on the immunoreactivity of calretinin in the superficial layers of the hamster superior colliculus (SC). In the present study, we describe the distributions and types of labeled cells and effects of enucleation in the deeper layers by immunocytochemistry. We also compare this labeling to that of GABA, the major inhibitory neurotransmitter in the central nervous system. In contrast to the superficial layers, the deeper layers contained many calretinin-IR neurons which formed two tiers. The first tier, which was very distinctive, was found within the intermediate gray layer. The second tier was found in the deep gray layer. Labeled neurons varied dramatically in morphology and included vertical fusiform, stellate, round/oval, and horizontal neurons. In contrast to the superficial layers, enucleation appeared to have no effect on the distribution of calretinin immunoreactivity in the deeper layers. Two-color immunofluorescence revealed that none of calretinin-IR neurons were labeled with an antibody to GABA. The present results demonstrate that calretinin identifies unique neuronal sublaminar organizations in the hamster SC. The present results also demonstrate that none of the calretinin-IR neurons in the hamster SC is GABAergic interneurons. As many calretinin-IR cells are GABAergic interneurons in most other brain areas, this phenomenon in hamster SC is exceptional.

Graded ephrin-A2 expression in the developing hamster superior colliculus

During development, ephrin gradients guide retinal ganglion cell axons to their appropriate topographic locations in the superior colliculus (SC). Expression of ephrin-A2, assessed immunohistochemically in the developing hamster SC, revealed a rostral(low) to caudal (high) gradient that is most prominent at postnatal days P4 and P7 when topography is established. Double-labelling immunohistochemistry for ephrin-A2 and cell specific markers revealed that ephrin-A2 is expressed exclusively by a subset of neurons. The expression pattern has implications for mechanisms underlying establishment of topography during development and following injury.

NMDA receptor blockade in the superior colliculus increases receptive field size without altering velocity and size tuning.

Neonatal brain injury triggers compensatory processes that can be adaptive or detrimental, but little is known about the mechanisms of compensation or how they might affect the response properties of neurons within the injured region. We have studied this issue in a rodent model. Partial ablation of the hamster superior colliculus (SC) at birth results in a compressed but complete visual field map in the remaining SC and a compensatory conservation of receptive field (RF) size and stimulus velocity and size tuning. The circuit underlying stimulus tuning in this system or its preservation after brain lesions is not known. Our previous work has shown that N-methyl-d-aspartate (NMDA) receptors are necessary for the development and conservation of RF size after partial SC ablation. In this study, we examined whether NMDA receptor function is also necessary for the development and conservation of stimulus velocity and size tuning. We found that velocity and size tuning were unaffected by chronic postnatal blockade of NMDA receptors and the resulting increases in RF size. Thus NMDA receptors in the SC are not necessary for the development of stimulus velocity and size tuning or in the compensatory maintenance of these properties following brain damage. These results suggest that stimulus velocity and size tuning may arise in the retina or from NMDA receptor-independent circuitry intrinsic to SC. The lack of conflict between NMDA receptor activity-dependent and -independent processes may allow conservation of some RF properties while others change during injury-induced or evolutionary changes in afferent/target convergence.

Morphology of Calretinin-immunoreactive Neurons in the Superficial Layers of Hamster Superior Colliculus after Enucleation: Lack of Co-Localization with GABA

Morphology of Calretinin-immunoreactive Neurons in the Superficial Layers of Hamster Superior Colliculus after Enucleation: Lack of Co-Localization with GABA

Changes of calretinin, calbindin D28K and parvalbumin-immunoreactive neurons in the superficial layers of the hamster superior colliculus following monocular enucleation

We studied the effects of monocular enucleation on the patterned distribution of calretinin-, calbindin D28K- and parvalbumin-immunoreactive (IR) neurons in the superficial layers of the hamster superior colliculus (SC). The calcium-binding proteins were localized using antibody immunocytochemistry. Almost complete depletion of the calretinin-IR fibers in the superficial layers of the contralateral SC was found following unilateral enucleation. Quantitative analysis showed that on the experimental side of the SC, an enormous number of calretinin-IR cells newly appeared (716%). On the experimental side of the SC, the number of parvalbumin-IR cells also increased (32%). By contrast, on the experimental side of the SC, the number of calbindin D28K-IR cells exhibited a reduction (43%). Two-color immunofluorescence revealed that none of the newly appeared calretinin-IR cells were labeled with antibodies to calbindin D28K or parvalbumin. The present results demonstrate that retinal projection may control the activity of the expression of these calcium-binding proteins in the hamster SC but in different manners. The results also show that the patterned change of calretinin and parvalbumin in the hamster SC is comparable with other animals, but the change of calbindin D28K is not identical.

Neonatally elevated serotonin levels alter terminal arbors of individual retinal ganglion cells in superior colliculus of hamsters.

Previous studies from this laboratory showed that sprouting of serotoninergic (5-HT) axons in the hamster's superior colliculus (SC), induced by a single subcutaneous injection of 5,7-dihydroxytryptamine (5,7-DHT) at birth (postnatal day 0 [P-0]), resulted in an increased terminal distribution of the uncrossed retinocollicular projection that was not associated with any changes in the number or distribution of ipsilaterally projecting retinal ganglion cells. The present study was undertaken to determine what effect this manipulation had on the terminal arbors of such axons. Retinocollicular axons of normal and 5,7-DHT-treated animals were anterogradely labeled with small intraretinal injections of the lipophilic dye 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI) on P-16. After tissue processing on P-19, single retinocollicular axon arbors were reconstructed by using confocal microscopy. Quantitative analysis indicated that arbors from 5,7-DHT-treated hamsters had significantly greater total fiber lengths, areas, and volumes than those from normal animals. There were no differences between axons from the two groups in number of branch points, distribution of relative branch lengths, and numbers of bouton-like swellings. These results support the hypothesis that increased SC concentrations of 5-HT alter development of the uncrossed retinocollicular pathway such that a greater territory is covered by individual terminal arbors but that the number of synaptic contacts per arbor remains constant. This may explain, at least in part, the abnormally widespread distribution of the aggregate ipsilateral projection.

Effects of norepinephrine on the activity of visual neurons in the superior colliculus of the hamster.

Single-unit recording and micropressure ejection techniques were used to test the effects of norepinephrine (NE) on the responses of neurons in the superficial layers (the stratum griseum superficiale and stratum opticum) of the hamster's superior colliculus (SC). Application of NE suppressed visually evoked responses by -30% in 75% of 40 neurons tested and produced > or = 30% augmentation of responses in only 5%. The decrement in response strength was mimicked by application of the alpha2 adrenoceptor agonist, p-aminoclonidine, the nonspecific beta agonist, isoproterenol, and the beta1 agonist, dobutamine. These agents had similar effects on responses evoked by electrical stimulation of the optic chiasm and visual cortex. The alpha1 agonist, methoxamine, augmented the light-evoked responses of 53% of 49 SC cells by > or = 30%, but had little effect on responses evoked by electrical stimulation of optic chiasm or visual cortex. The effects of adrenergic agonists upon the glutamate-evoked responses of SC cells that were synaptically "isolated" by concurrent application of Mg2+ were similar to those obtained during visual stimulation. Analysis of effects of NE on visually evoked and background activity indicated that application of this amine did not significantly enhance signal-to-noise ratios for most superficial layer SC neurons, and signal-to-noise ratios were in some cases reduced. These results indicate that NE acts primarily through alpha2 and beta1 receptors to suppress the visual responses of SC neurons. Activation of either of these receptors reduces the responses of SC neurons to either of their two major visual inputs as well as to direct stimulation by glutamate, and it would thus appear that these effects are primarily postsynaptic.

Augmentation of serotonin in the developing superior colliculus alters the normal development of the uncrossed retinotectal projection

A previous study from this laboratory showed that sprouting of serotoninergic axons in the hamster's superior colliculus (SC) induced by a single subcutaneous injection of 5,7-dihydroxytryptamine (5,7-DHT) at birth (postnatal day 0; P-0) resulted in an abnormal terminal distribution of the uncrossed retinotectal projection. The present study provided further evidence to support the role of increased 5-HT levels within the SC in this phenomenon. Slow-release polymer (ELVAX) chips impregnated with serotonin (5-HT) were placed over the SC on either P-1 or P-3, and retinotectal projections were assessed via anterograde transport of horseradish peroxidase when animals reached P > 18. Analysis of ELVAX chips indicated that they released 5-HT in amounts of > or = 1 pmole/hour for at least 12 days. Assessment of the SC of treated hamsters indicated significantly elevated 5-HT concentrations as late as P-12, but not on P-16. Implantation of 5-HT chips, but not control chips, resulted in abnormalities in the uncrossed retinotectal projection similar to those observed in the 5,7-DHT-treated animals. The patches that normally develop in the rostral part of the stratum opticum were not present, and uncrossed axons were distributed densely in this layer and in the lower portion of the stratum griseum superficiale throughout the rostrocaudal and mediolateral extents of the SC. Quantitative analysis of these changes indicated significant differences between the organization of the uncrossed retinotectal projections of 5-HT-treated animals vs. either blank-implant treated or completely untreated animals but not between 5-HT-treated hamsters and animals that received neonatal 5,7-DHT injections. All of these results support the conclusion that increased SC concentrations of 5-HT altered retinotectal development.

Effects of neonatal and adult enucleation on the synaptic organization of the serotoninergic projection to the superficial gray layer of the hamster's superior colliculus

Previous experiments from this laboratory have demonstrated that either neonatal or adult enucleation results in a marked increase in the density of the serotoninergic (5-HT) projection to the superficial layers of the hamster's superior colliculus (SC). The present study was undertaken to determine whether this alteration in the density of the 5-HT projections to SC in hamsters that sustained eye removals was associated with any change in the synaptic organization of this pathway. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles made synaptic contacts in the stratum griseum superficiale (SGS). Of these, 65% were axoaxonic, 30% were axodendritic, and 5% were axosomatic. In contrast, 22% of 400 5-HT-positive profiles in the neonatally enucleated hamsters and 17.5% of 400 such profiles in hamsters that sustained eye removals in adulthood made synaptic contacts. In the neonatal enucleates, 66% of the contacts were axoaxonic, 33% were axodendritic, and 1% were axosomatic. In the adult enucleates, 73% of the contacts were axoaxonic, 26% were axodendritic, and 1% were axosomatic. Thus, both neonatal and adult enucleation increase the percentage of 5-HT-positive profiles that make synaptic contacts in SC, but neither manipulation significantly changes the distribution of these contacts among different types of postsynaptic targets.

Neonatal administration of the neurotoxin 5,7-dihydroxytryptamine results in synaptic reorganization in the superficial gray layer of the hamster's superior colliculus.

Neonatal subcutaneous administration of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) to hamsters results in a marked depletion of serotonin (5-HT) in cortex and an increase in the concentration of this amine in the superior colliculus (SC). To determine whether this increase was associated with an alteration in the synaptic organization of 5-HT-containing axons in the superficial gray layer of the SC, immunocytochemistry was combined with electron microscopy. In normal adult hamsters, only 4.0% of 500 5-HT-immunoreactive profiles make synaptic contacts in the superficial gray layer of the hamster's SC. In 5,7-DHT-treated animals, examination of 400 individual profiles indicated that 25.5% of 5-HT-positive profiles made synaptic contacts (P < 0.05). Given the recently demonstrated effect of 5-HT on retinotectal transmission in this species, the present results suggest that the functional organization of the SC may also be markedly altered in animals that sustain neonatal 5,7-DHT administration.

Ultrastructural organization of the noradrenergic innervation of the superficial gray layer of the hamster's superior colliculus

Immunocytochemistry with an antibody-directed tyrosine hydroxylase (TH) was combined with electron microscopy and serial-section analysis to examine the synaptic organization of the catecholaminergic projection to the stratum griseum superficiale (SGS) of the hamster's superior colliculus (SC). A total of 250 TH-immunoreactive profiles within SGS were examined. Of these, 114 (45.6%) made synaptic contacts; 81 (71.1%) were axodendritic, and the remainder (33, 28.9%) were axo-axonic. Serial-section analysis was employed to evaluate the presence or absence of synaptic contacts for 26 profiles. Overall, 19 (73.1%) of the profiles followed through serial sections exhibited synaptic contacts. Double staining of single sections with antibodies directed against TH and dopamine-beta-hydroxylase (D beta H) and examination in the light microscope indicated that virtually all TH-positive fibers also contained D beta H. This indicated that the fibers examined at the electron microscopic level were noradrenergic rather than dopaminergic. These results suggest that norepinephrine may have both pre- and postsynaptic actions in the hamster's SC and that at least some of these effects are mediated by conventional synapses.

Effects of illumination and enucleation on substance-P-immunoreactive structures in subcortical visual centers of golden hamster and Wistar rat.

The undecapeptide substance P is found in different entities of the visual system that control eye movement and synchronize endogenous rhythms with the light cycle (i.e., superior colliculus, suprachiasmatic nucleus, intergeniculate leaflet). Immunocytochemical methods were used to compare the reactivity to substance P in the brain of five groups of golden hamsters and two groups of Wistar rats: (1) untreated hamsters kept under 14L:10D and sacrificed at noon; (2) identically maintained animals sacrificed at midnight; (3) enucleated animals kept under control conditions; (4) hamsters kept under constant darkness; (5) hamsters kept under the same conditions as the controls, but intraventricularly injected with colchicine. The results obtained in golden hamsters of groups (1) and (3) were compared with findings in Wistar rats treated accordingly [groups (6) and (7)]. Substance P-immunoreactive perikarya were found in the suprachiasmatic nucleus and superior colliculus of hamsters and Wistar rats. Substance P-immunoreactive nerve fibers were abundant in the hypothalamic area ventral to the paraventricular nucleus, in the intergeniculate leaflet, in some thalamic nuclei, and in the superior colliculus. Immunoreactivity to substance P in the suprachiasmatic nucleus and intergeniculate leaflet did not vary among the experimental groups. However, a conspicuous decrease in reactivity to substance P was observed in the superficial layers of the superior colliculus of enucleated hamsters and rats, compared with all other groups. These results indicate that substance P immunoreactivity in the superior colliculus, but not that in the suprachiasmatic nucleus or intergeniculate leaflet, depends on the integrity of the retinal projection.

Correlations between the receptive field properties and morphology of neurons in the deep layers of the hamster's superior colliculus

Extracellular and intracellular recording, receptive field mapping, and intracellular HRP injection techniques were used to define the morphological classes of cells in the deep laminae of the hamster's superior colliculus and to determine whether there are any correlations between the structural and functional characteristics of these neurons. A total of 110 neurons were characterized and reconstructed. Of these, 23.6% (N = 26) were visual, 60% (N = 66) were somatosensory, 0.9% (N = 1) were bimodal (visual-somatosensory), and 15.4% (N = 17) were unresponsive. Of the somatosensory neurons, 72.7% (N = 48) were low threshold, 4.5% (N = 3) had a wide dynamic range, 9.1% (N = 6) responded only to noxious stimulation, and 13.6% (N = 9) had complex somatosensory receptive fields. Deep layer cells were divided into eight morphological classes. These classes were multipolar cells (26.4%, N = 29), bipolar cells (9.1%, N = 10), widefield vertical cells (7.3%, N = 8), horizontal cells (13.6%, N = 15), stellate cells (10.9%, N = 12), ventrally directed cells (5.5%, N = 6), sparse radial cells (17.3%, N = 19), and small sparse radial cells (6.4%, N = 7). Four cells (3.6%) did not fit into this classification scheme. Univariate and multivariate analyses of variance of properties such as soma area, number of branch points, total dendritic length, and volume and orientation of dendritic arbor indicated that these classes were significantly different. However, chi 2 analysis and multivariate analysis of variance indicated no significant relationships between morphological class and either laminar location or receptive field type. There was a significant positive relationship between the possession of dendrites that extended into the superficial laminae and visual responsivity.

Reduction in the percentage of serotoninergic axons making synapses during the development of the superficial layers of the hamster's superior colliculus

Immunocytochemistry with an antibody directed against a serotonin (5-HT)-bovine serum albumin conjugate was combined with electron microscopy to assess the synaptic organization of the serotoninergic projection to the stratum griseum superficiale (SGS) of the superior colliculus of hamsters killed on postnatal days (P) 0, 3, 7, 12, 15, 20 and > 60. At least 200 5-HT-immunoreactive profiles were examined at each of these ages. In the newborn (PO) animals, 36.6% of the 5-HT-positive profiles made conventional synapses. This percentage remained fairly constant until P15 when only 17.3% of the profiles made conventional synaptic contacts. On P20, this value decreased to 10.5% and in the adult animals, it fell to 4.4% These results thus indicate a dramatic age-related change in the synaptic organization of the 5-HT input to the hamster's superior colliculus.

Increased serotoninergic innervation of the hamster's superior colliculus alters retinotectal projections.

Anterograde tracing with horseradish peroxidase (HRP) was used to compare the organization of retinotectal projections in normal adult hamsters and in animals that sustained subcutaneous injections of the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) on the day of birth. Neonatal injection of this neurotoxin decreases the density of the serotoninergic (5-HT) innervation of the cerebral and cerebellar cortices, but increases the density of these fibers in the brainstem including the superior colliculus (SC). Analysis of tissue from the retinorecipient laminae of the SC by high-pressure liquid chromatography indicated that these lesions increased the amount of 5-HT in the adult SC by 47%. The increased serotoninergic innervation of SC was associated with a marked change in the distribution of the uncrossed retinotectal projection. In normal adult hamsters, fibers from the ipsilateral eye form dense clusters in the lowermost stratum griseum superficiale (SGS) and stratum opticum (SO). A small number of uncrossed fibers are also visible in the more caudal portions of these layers. In the animals that sustained neonatal 5,7-DHT injections, uncrossed retinotectal fibers formed a nearly continuous band in rostral SO and lower SGS, and numerous labeled fibers were present in the caudal SC, primarily in the SO. Neonatal treatment with 5,7-DHT also produced alterations in the crossed retinotectal pathway and in the crossed and uncrossed retinogeniculate projections. These results indicate that the 5-HT input to the developing brainstem may strongly influence the development of retinofugal projections.

Effects of serotonin on retinotectal-, corticotectal-, and glutamate-induced activity in the superior colliculus of the hamster

1. Single-unit recording and iontophoretic techniques were used to test the effects of serotonin (5-HT) on the responses of neurons in the superficial layers (the stratum griseum superficiale and stratum opticum) of the hamster's superior colliculus (SC). 2. Iontophoresis of 5-HT produced a visual response suppression of 40% or greater in 78.1% (n = 50) of 64 neurons tested. 5-HT did not augment the visual responses of any of the cells tested. The average response suppression was 75.3 +/- 21.2% (mean +/- S.D.). 3. Iontophoresis of 5-HT had significantly different effects on activation of SC cells by optic chiasm (OX) and visual cortical (CTX) stimulation. Application of 5-HT suppressed the OX-evoked responses of 96.9% (n = 31) of the 32 SC cells tested by at least 40%, and the average response suppression for all 32 neurons tested was 87.1 +/- 22.5%. Application of 5-HT suppressed the responses of only 35.7% (n = 10) of the 28 cells tested with CTX stimulation by at least 40%. The average response suppression for all 28 cells was 35.3 +/- 38.8%. 4. The effects of 5-HT on the glutamate-evoked responses of SC cells that were synaptically "isolated" by concurrent application of Mg2+ were also evaluated. Application of 5-HT produced a response suppression > or = 40% in 29.7% (n = 19) of the 64 neurons tested under these conditions. The average response suppression for all of the cells tested was 28.4 +/- 35.7%. This effect of 5-HT was significantly weaker than that on visually evoked responses of these neurons. 5. The present results demonstrate that 5-HT markedly depresses the visual responses of most superficial layer SC neurons. They suggest further that much of this effect is mediated by presynaptic inhibition of retinotectal transmission.

Changes in synaptic density after developmental compression or expansion of retinal input to the superior colliculus

The retinal projection to the superior colliculus can be made abnormally dense by inducing a "compressed" retinal projection into a subnormal tectal volume, or abnormally sparse by monocular enucleation early in development. Any or all of the features of cell number, axonal arbor, dendritic arbor, and synaptic density could potentially be adjusted to compensate for such variations in the convergence of one cell population on another. We have examined the consequences of neonatal partial tectal ablation or monocular enucleation for synaptic length, density, and relative numbers of synapse classes in the superficial gray layer of the hamster superior colliculus. Monocular enucleation resulted in a reduction of synaptic density in the superficial gray layer of the colliculus ipsilateral to the remaining eye. This decrease in density was entirely accounted for by a reduction of the number of synapses with round vesicles, large asymmetric terminal specializations, and pale mitochondria characteristic of retinocollicular terminals (RLP synapses). There was no compensatory increase in any other synaptic class. RLP synapses were larger in monocular enucleates. Partial tectal ablation had no effect on synaptic density, nor on the relative proportions of different synaptic types. Synapses of the RLP class were slightly smaller than normal. These results suggest that synaptic density is normally at a maximum that cannot be altered by increases in potential input. However, density may be reduced by decreasing the number of inputs. Terminal classes do not appear to compete with each other within the collicular volume, suggesting that postsynaptic cells controls both the classes and numbers of their potential inputs.

Chapter 10 Early and late flash-induced field responses correspond to ON and OFF receptive field components in hamster superior colliculus

Publisher Summary The chapter discusses that early and late negative waves, which were induced by diffuse flash in the hamster superior colliculus (SC), correspond respectively to summated unit activities of ON and OFF components. Observations to support this conclusion are three-fold. First, after stationary light stimuli of certain duration two negative waves that are similar to those induced by diffuse flash, appear in the SC at light ON and OFF. Second, the early negative wave disappears after blockade of the ON pathway by intravitreal APB application, while the late one remains unchanged. Third, response latencies of the late negative wave of the SC field response become longer at shorter duration of light, as those of OFF discharges of OFF center cells and ON-OFF center cells. The finding that the OFF latencies are significantly longer than ON latencies at brief light are explained by the different time course of ON and OFF bipolar cells to brief light. ON bipolar cells in the retina depolarize to light, while OFF bipolar cells hyperpolarize to the same stimulus.